WASHINGTON — A Texas water agency is claiming an “unprecedented” right to take water from Oklahoma, but a decades-old agreement prohibits the diversion, an attorney for Oklahoma told U.S. Supreme Court justices here Tuesday.

Washington attorney Lisa Blatt, who is representing Oklahoma in the dispute, said no water agreement among states has ever allowed one state to reach into another without explicit provisions that spell out all of the conditions.

The attorney for the Tarrant Regional Water District, which supplies water to residents in north central Texas, told the justices that Oklahoma had agreed in the Red River Compact that Texas has a right to certain water in Oklahoma.

“Oklahoma is now trying to back out of that bargain,” Charles A. Rothfeld said.

The Supreme Court is expected to decide sometime this summer whether Oklahoma or the Texas agency is right in a lawsuit dating to 2007, and the decision likely will hinge on how justices read a provision in the compact that divides excess water from the Kiamichi River in Oklahoma.

The provision gives the four states that signed the compact — Oklahoma, Texas, Arkansas and Louisiana — equal rights to the excess water and caps the shares at 25 percent.

However, it doesn't specifically say whether one state can cross a border to get its share.

A federal district court and the 10th U.S. Circuit Court of Appeals has agreed with Oklahoma in the case that Texas can't cross the border to get its share.

The Tarrant water district says the allocation of water in the provision isn't based on state boundaries; rather, Rothfeld told the justices on Tuesday, the water at issue is “a common pool of water” defined by dam sites.

Seeking clarity

Justice Ruth Bader Ginsburg told Rothfeld that other water compacts clearly express when a state can cross into another to get water.

“This clause, the one that you rely on, is kind of sketchy, isn't it?” Ginsburg asked Rothfeld. “Doesn't say how they're going to get it, if they're going to pay for it. There's a lot to be filled in.”

Rothfeld replied that the provision was “quite clear” in allowing all four states equal access to the water.

Justices seemed at times perplexed and ultimately worn out by the vagaries of the Red River Compact, which divides the basin into “reaches” and “sub-basins” and allots water among the states in various ways — sometimes with specific reference to state boundaries and sometimes not.

The justices looked at maps and read through various parts of the Red River Compact — which was approved by Congress in 1980 — and sometimes based their questions on parts of the compact that aren't in dispute.

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Chris Casteel began working for The Oklahoman's Norman bureau in 1982 while a student at the University of Oklahoma. After covering the police beat, federal courts and the state Legislature in Oklahoma City, he moved to Washington in 1990, where...